1. Field of the Invention
The present invention relates generally to improvements in apparatus and methods for retrieval of stored data from a disc drive data track, and more particularly, but not by way of limitation, to improvements in timing and gain control in the retrieval of data in disc drives having a PRML read channel.
2. Brief Description of the Prior Art
In a disc drive, user files are stored along concentric data tracks defined in magnetizable surface coatings on the surfaces of rotating discs. To this end, during storage of a file, user data is encoded and bits of the encoded file are serially clocked to a write head driver that passes an electrical current through a write head adjacent a selected disc surface to magnetize segments of a selected data track in a pattern that reflects the sequence of logical values of bits that comprise the encoded file. These magnetized segments, in turn, produce a magnetic field that can be sensed by a read head during reading to generate a sequence of electrical pulses that reflects the pattern of magnetization of the data track to permit recovery of the encoded file for decoding and return to a computer which makes use of the disc drive.
Conventionally, the electrical pulses induced in the read head, each corresponding to a flux transition along the data track; that is, a reversal in the direction of magnetization of the surface coating, have been peak detected within "read windows" that are generated by a read clock and the read clock also provides clock signals that are employed in transferring logic values, corresponding to the presence or absence of peaks in the read windows, to downstream components which regenerate the user file and make the file available to the computer. Synchronization between the generation of pulses in the read head and generation of clock signals by the read clock, necessary to transfer each logic value to downstream components as it is produced, has been effected using a Phase Locked Loop (PLL) for the read clock and transmitting pulse edges generated in the peak detector to the phase locked loop while data is being read from a data track. Thus, the read clock has been continually readjusted to track the electrical pulses induced in the read head by passage of flux transitions along a data track by the read head.
The synchronization between the generation of logic values from pulses induced in the read head and the generation of clock pulses used to transfer the logic values into downstream components in a conventional disc drive permits the use of high transfer rates in the writing and subsequent reading of data to, in turn, provide a large file storage capacity for the disc drive and much work has been done to maximize the data storage capacity of such disc drives via increases in the transfer rate. However, several effects tend to limit transfer rate. The synchronization depends upon a correspondence between peaks in the signal induced in the read head and passage of individual flux transitions by the read head. However, the magnetic field from which the read head signal is derived is a superposition of the magnetic fields produced by all of the flux transitions on the disc. Consequently, as the transfer rate is increased to decrease the spacing of flux transitions along a data track, so-called "intersymbol interference"; that is, significant superposition of magnetic fields from successive flux transitions on a data track, causes peaks in the read head to be shifted in time from the times that such peaks would occur for an isolated flux transition. While the effects of intersymbol interference can be minimized; for example, by pulse slimming and prewrite compensation, it cannot be eliminated. As a result, as transfer rates have been increased, peak shifting has become an increasingly difficult problem that has tended to limit transfer rates in conventional disc drives.
A second effect that gives rise to a limitation in the transfer rate in disc drives employing a peak detect read channel is electronic noise in the read channel. As the transfer rate is increased, it becomes increasingly difficult to filter high frequency noise that can cause errors in the retrieval of a file from the signals induced in the read head. Consequently, while high data transfer rates have been achieved in disc drives employing peak detection to recover stored data, it has become increasingly difficult to achieve further increases in the transfer rate and, in turn, the data storage capacity, of such drives.
Because of this difficulty in increasing the data transfer rate in disc drives employing peak detection in the read channel, increasing interest has arisen in recent years in disc drives having so-called PRML read channels. In disc drives of this type, partial response signalling is utilized to control, rather than to suppress, intersymbol interference and the effect of noise is minimized by the use of maximum likelihood estimation of the magnetization of sequences of segments of the data track. To this end, signals corresponding to individual flux transitions are filtered to a signal which, in the absence of noise, would have a nominal form, for which the intersymbol interference is known and can be controlled, and the signals are then sampled for maximum likelihood detection in which each bit of encoded data is recovered in the context of the sequence of bits that were written to the disc to limit the effect of noise. Since the intersymbol interference can be taken into account in the regeneration of the data and since the data is recovered in relation to sequences of bits, disc drives utilizing PRML read channels have a potential for achieving transfer rates that will greatly exceed transfer rates that can be achieved in disc drives that employ peak detection of signals induced in the read head.
In practice, problems have arisen with the use of PRML read channels that limit transfer rates that can be achieved. Since samples of the filtered signal are used to determine the sequence of data bits written to a data track, the samples must be taken at specific times determined by the nominal form to which the signals in the channel are, ideally, filtered and the amplitudes of the signals must be carefully controlled if intersymbol interference is to be appropriately compensated and maximum likelihood detection is to yield the sequence of data bits that were written to the disc. More specifically, differences between actual sample times and sample times dictated by the nominal signal wave form and channel gain variations act as noise which can interfere with the ensuing maximum likelihood detection of the signals. Thus, an important aspect of the use of a PRML read channel in a disc drive is control of the timing of clock signals used to sample signals in the read channel and to operate the maximum likelihood detectors that recover the sequence of stored data bits and control of the amplitude of read channel signals.
The general approach that has been taken to sampling time and signal amplitude control in disc drives having PRML read channels is based on theory that has been presented in the paper: K. H. Mueller and M. Muller, "Timing Recovery in Digital Synchronous Data Receivers", IEEE Transactions on Communications, Vol. COM-24, pp 516-530, May 1976. Applying this theory to a partial response type 4 channel (PR-4) in which the present invention is used, a time error signal is generated at each sampling interval k in accordance with the equation: EQU time error (k)=x(k) y(k-1)-x(k-1) y(k), (1)
where x(k) is the estimate of the presence or absence of a legitimate nonzero sample and its polarity at the kth sampling moment, y(k) is the actual sample value for the kth moment and x(k-1) and y(k-1) are similarly defined for the previous sampling moment. Estimates are practically obtained by the comparison of the sample with a threshold set to one half the amplitude of the nominal signal to which signals in the channel are ideally filtered; that is: EQU Threshold=.+-.A/2, (2)
where A is the amplitude of the nominal signal. For the purpose of the present invention, the estimates have only three possible values: -1, 0, and +1.
The time error signal so determined is then used to adjust the frequency and phase of a voltage controlled oscillator that generates clock signals used to time the sampling and the operation of the maximum likelihood detection circuitry.
Similarly, a gain error, used to adjust the gain of a variable gain amplifier in the read channel to control the amplitude of the filtered signal, can be developed in accordance with the relation: EQU gain error (k)={y(k)-A sign [x(k)]} x(k)+{y(k-1)-A sign [x(k-1)]} x(k-1). (3)
While the underlying theory of time and gain control in a PRML disc drive is thus well known, practical problems in the application of the theory have limited the degree to which such control can be maintained with increasing transfer rates so that the promise of higher transfer rates that underlies the development of PRML read channels has been largely unrealized. As in the case of peak detection circuitry, characteristics of components utilized to realize a timing circuit in a disc drive having a PRML read channel are limited by economics to, in turn, limit the extent to which sampling times and signal amplitudes of the signals can be controlled. While this problem can, to some extent, be overcome by digitizing the samples and utilizing digital circuitry, a digital filter and digital signal processing to implement the above expressions, the use of digital signal processing introduces new problems. The digital signal processing requires at least several additional clock cycles. This, introduces additional processing time in the timing circuitry that is unrelated to the sample time indicated by the theory with the result that time and gain corrections are delayed with respect to the times at which the samples are taken. Such delay is usually termed a "transportation lag" or a "dead time" in control theory. See, for example, K. Ogada, "Modern Control Engineering", Prentice-Hall, Englewood Cliff, N.J., 1970, pp 346-350. The dead time adversely affects PLL performance and can easily cause instability of the loop.